Conductive agent for battery electrode, electrode containing the same, and battery

ABSTRACT

Disclosed is a highly reliable secondary battery, as well as an electrode and a conductive agent used therefor, which battery has a long cycle life and is also less likely to be damaged or rupture even when the battery temperature becomes abnormally high. The conductive agent of the battery electrode contains, as the main component, a reaction product between a π-conjugated carbon material and a soluble polyimide, preferably a soluble block copolymerized polyimide. The battery electrode is formed by coating a composition containing this conductive agent and an electrode active substance onto a current collector. The battery comprises this electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of copending application Ser. No.12/993,729 filed on Apr. 21, 2011, which is the U.S. National Phase ofPCT/JP2009/059190, filed May 19, 2009, and which claims priority toApplication No. 2008-134011 filed in Japan, on May 22, 2008. The entirecontents of all of the above applications are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to a conductive agent for electrodes ofbatteries such as lithium batteries, as well as to an electrodecontaining the conductive agent and a battery.

BACKGROUND ART

In recent years, as a negative electrode material for lithium secondarybatteries, carbon materials such as coke and graphite have been proposedto replace conventional lithium metals because, for example, they havesuperior flexibility and are not likely to cause electrodeposition ofmossy lithium.

A negative electrode using the aforementioned carbon material is usuallyprepared by, for example, a method in which carbon powder (such asgraphite and coke powder) and, as required, conductive agent powder(such as acetylene black and carbon black) are dispersed in a bindingagent solution to produce a slurry, which is then coated onto a currentcollector metal by the doctor-blade method and subsequently dried.

Thus, conventionally, as the binding agent solution, a solution in whichPVDF (polyvinylidene difluoride) is dissolved into NMP(N-methyl-2-pyrrolidone) has been used.

However, although PVDF is an excellent binding agent for integratingcarbon powder, since its adhesiveness (adhesion) with a currentcollector metal is poor, the carbon powder detaches from the currentcollector metal (such as a copper plate and a copper foil) by repeatedcharge and discharge, thereby gradually lowering the battery capacity.That is, those batteries using PVDF have a problem in that their cyclelife is generally short. The same tendency is observed also in therelation between a positive electrode active substance and the bindingagent.

In addition, PVDF not only allows stable adhesion of those carbonpowders themselves used as a negative electrode active substance andpositive electrode active substances themselves, but also stably adheresthose conductive agents blended in positive and negative electrodes, aswell as a conductive agent with an active substance, thereby maintaininga balance between electrons and ions within a battery. Their adhesion(attachment) and dispersibility are consequently poor; therefore, inorder to attain smooth charge and discharge, it is necessary to blend alarge amount of conductive agent at the expense of the battery capacity.Moreover, since the conductive agent per se is a foreign matter to theactive substance, mechanical bonding between the active substance andthe conductive agent, and blending a large amount of the activesubstance and conductive agent cause drawbacks such as corrosion andcycle deterioration.

PRIOR ART DOCUMENTS Patent Documents

-   -   [Patent Document 1] JP 9-265990 A    -   [Patent Document 2] JP 3561701 B

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention was made in view of the above circumstances and anobject thereof is to provide a highly reliable battery, as well as anelectrode and a conductive agent used therefor, which battery has a longcycle life and is also less likely to be damaged or ruptured even whenthe battery temperature becomes abnormally high.

Means for Solving the Problems

The present inventors intensively studied to discover that theaforementioned object of the present invention can be attained byforming the conductive agent, which constitutes an electrode, from acomposition containing, as the main component, a reaction productbetween a π-conjugated carbon material and a soluble polyimide, therebycompleting the present invention.

That is, the present invention provides a conductive agent for batteryelectrodes, comprising as main component a reaction product between aπ-conjugated carbon material and a soluble polyimide. The presentinvention also provides a battery electrode, which is formed by coatinga composition containing the conductive agent according to the presentinvention and an electrode active substance onto a current collector.The present invention further provides a battery comprising theelectrode according to the present invention.

Effects of the Invention

According to the present invention, provided for the first time are ahighly reliable secondary battery, as well as an electrode and aconductive agent used therefor, which battery has a long cycle life andis also less likely to be damaged or ruptured even when the batterytemperature becomes abnormally high. The conductive agent according tothe present invention is characterized by containing, as the maincomponent, a substance obtained by allowing a carbon material which islikely to cause separation or detachment phenomenon to react with asoluble polyimide. Thus, since the conductive agent and binder, as wellas active substance and the like disperse without an energy barrier, areduction in the specific resistance and interfacial resistance of theelectrodes are observed. In addition, the charge and dischargeefficiency is high and the battery capacity hardly decreases even withrepeated cycles; therefore, the cycle life is long as well. Furthermore,since the soluble polyimide used in the present invention is believed tosuppress redox-type oxidation reactions and radical chain reactions,inconveniences such as battery thermal runaway, smoking, ignition andexplosion may be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of the lithium secondarybattery prepared in Examples of the present invention.

FIG. 2 shows the measurement results of the impedance in the planardirection in dried condition for each of the positive electrodes of thebatteries prepared in an Example and Comparative Example.

MODE FOR CARRYING OUT THE INVENTION

As described above, the conductive agent according to the presentinvention contains, as the main component, a reaction product between aπ-conjugated carbon material and a soluble polyimide.

Generally, π-conjugated carbon materials are also referred to as C6compounds containing a π-bond. They are composed of carbon and contain astructure which forms an electron-conductive orbital band by coupling ofthe carbon π-electrons, in which structure 6-membered rings of carbonatoms are condensed in such a manner that regular hexagons areadjacently arranged one another (a small number of 5-membered and/or7-membered rings may be present). Examples of preferred π-conjugatedcarbon material used in the present invention include Ketjen Black,acetylene black, carbon nanotube and other carbon materials having aprimary particle diameter of not greater than 10 nm, and particularlypreferred are Ketjen Blacks, acetylene blacks and carbon nanotubes.Here, specific examples of the aforementioned “other carbon materialshaving a primary particle diameter of not greater than 10 nm” include,for example, fullerene and carbon nanohorns. Since the aforementionedπ-conjugated carbon materials are all commercially available, acommercially available product may be used. Also, such π-conjugatedcarbon materials may be used individually or two or more of them may beused in combination.

As described above, in the π-conjugated carbon materials, anelectron-conductive orbital band is formed by coupling of the carbonπ-electrons. Consequently, as a substance, the surface energy isextremely high; therefore, not only there arise problems, for example,that the conductive agent is likely to aggregate each other; that theadhesion between the active substance and current collector is low; andthat the viscosity of slurry prepared to be coated onto electrodes isexcessively increased, but also it is necessary to blend a large amountof conductive agent in order to attain the desired conductivity. In viewof this, in order to mitigate the high surface energy of theaforementioned π-conjugated carbon materials, the present inventorssearched for a substance capable of reacting with the surface ofπ-conjugated carbon material at a level where at least a complex can beformed, and discovered a soluble polyimide. Therefore, the conductiveagent according to the present invention contains a soluble polyimide asan essential constituting component.

The soluble polyimide used in the present invention is a polyimide whichis soluble to a nitrogen-containing polar solvent such asN-methyl-2-pyrrolidone (NMP). Here, being soluble means that thepolyimide is dissolved in 100 g of a solvent at an amount of not lessthan 5 g. Examples of the polyimide used in the present inventioninclude those soluble polyimides whose glass transition temperature isnot higher than 270° C., more preferably not higher than 250° C., andsuch a soluble polyimide is preferably one which contains at least onecomponent having a carbonyl group and/or an ether group as aromaticring-linking group in the molecular skeleton. The lower limit of theglass transition temperature is not particularly restricted; however, itis usually not lower than 120° C. A lower glass transition temperaturecauses softening of the polyimide in the event where the batterytemperature becomes high, such as thermal runaway or the like;therefore, there is a concern from the safety standpoint.

A polyimide can be obtained by dehydration-condensation reaction of atetracarboxylic dianhydride and diamine. Examples of raw materialshaving a carbonyl group as the aromatic ring-linking group include3,3′,4,4′-benzophenonetetracarboxylic dianhydride,3,3′-diaminobenzophenone, 4,4′-diaminobenzophenone,4,4′-bis(4-aminophenoxy)benzophenone and the like. Examples of rawmaterials containing an ether group includebis(3,4-dicarboxyphenyl)ether dianhydride, 3,4′-diaminodiphenyl ether,4,4′-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene,1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene,4,4′-bis(4-aminophenoxy)biphenyl,2,2-bis[4-(4-aminophenoxyl)phenyl]propane,2,2-bis[4-(4-aminophenoxyl)phenyl]hexafluoropropane andbis[4-(3-aminophenoxyl)phenyl]sulfone,bis[4-(4-aminophenoxyl)phenyl]sulfone and the like. Further, a rawmaterial containing neither a carbonyl group nor an ether group, forexample, pyromellitic dianhydride, 3,3′,4,4′-biphenyltetracarboxylicdianhydride, diphenylsulfone-3,3′,4,4′-tetracarboxylic dianhydride,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride,2,4-diaminotoluene, 4,4′-diamino-3,3′-dimethyl-1,1′-biphenyl,4,4′-diamino-3,3′-dihydroxy-1,1′-biphenyl, 3,3′-diaminodiphenylsulfone,4,4′-diaminodiphenylsulfone, 3,5-diaminobenzoic acid,2,6-diaminopyridine, 2,6-diamino-4-methylpyridine,3,3′-diaminodiphenylmethane, 4,4′-diaminodiphenylmethane,3,3′-dicarboxy-4,4′-diaminodiphenylmethane, 4,4′-benzanilide,4,4′-(1,3-phenylenediisopropylidene)bisaniline,4,4′-(1,4-phenylenediisopropylidene)bisaniline,4,4′-(9-fluorenylidene)dianiline or5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane, may be used as onecomponent of the copolymer composition. Moreover, an aliphatic compoundsuch as cyclohexane-2,3,5,6-tetracarboxylic dianhydride orbicyclo[2.2.2]octo-7-ene-2,3,5,6-tetracarboxylic dianhydride and adiaminosiloxane compound such as bis(γ-aminopropyl)polydimethylsiloxanecan be used in combination; however, when their proportion is increased,it is observed that the heat resistance and durability tend to bereduced. Here, the soluble polyimide used in the present invention mayalso be a copolymer which contains at least two or more of therespective aforementioned varieties of acid components and aminecomponents, or a copolymer which contains one of either the acidcomponent or amine component and two or more additional components otherthan this, and it is preferred that the soluble polyimide be a blockcopolymer.

With regard to the method of synthesizing the polyimide, any knownmethod can be employed and it is not particularly restricted. Adiisocyanate method in which a corresponding diisocyanate compound isused in place of the aforementioned diamine compound may be employed;however, it is preferred that a chemical imidization method using acatalyst such as acetic anhydride/triethylamine orγ-valerolactone/pyridine be employed. A soluble polyimide varnish can beobtained by loading into a reaction vessel the above-exemplified atetracarboxylic dianhydride(s), a diamine compound(s),N-methyl-2-pyrrolidone (NMP) which is a solvent, an imidization catalystand a dehydrating agent and subsequently allowing them to react withstirring for several hours under a nitrogen atmosphere at a temperatureof 160 to 200° C. The mixing ratio of the tetracarboxylic dianhydrideand the diamine is preferably 0.95 to 1.05 mol % of the total diamineamount with respect to 1 mol % of the total acid dianhydride amount. Itis preferred that the solvent be a nitrogen-containing solvent, such asN,N-dimethylacetamide or NMP, which has a good dispersibility for theaforementioned conductive agent and a high solubility also forpolyimides, and it is more preferred that the solvent be NMP. Further,the dehydrating agent is used for removing water generated during thereaction from the reaction system, and a solvent which is azeotropedwith water may be used as the dehydrating agent. The dehydrating agentis preferably toluene and/or xylene and/or ethylcyclohexane. Thisdehydrating agent removes water with heat-reflux during the reactionsuch that no water remains in the varnish after completion of thereaction. A soluble polyimide prepared in this manner has aweight-average molecular weight of preferably 30,000 to 200,000, morepreferably 40,000 to 150,000. When the weight-average molecular weightis not higher than 30,000, the flexibility and the mechanical strengthtend to be impaired, while when it is not less than 200,000, the varnishviscosity tends to be high, thereby causing a problem in the step ofdispersing the conductive agent. The term “solvent-soluble” used hereinis applied to nitrogen-containing systems, such as NMP, where theconductive agent is easily dispersed. Considering the safety at the timeof battery production, it is preferred that the polyimide be insolubleto other solvents.

As described above, the polyimide used in the present invention has aglass transition temperature of preferably not higher than 270° C., morepreferably not higher than 250° C., and the polyimide is preferably asoluble polyimide which contains at least one component having acarbonyl group and/or an ether group as aromatic ring-linking group inthe molecular skeleton. When the glass transition temperature is notlower than 270° C., the polyimide tends to be rigid and fragile as aresin. In order to make the glass transition temperature of a solublepolyimide to be not higher than 270° C., preferably not higher than 250°C., such a method in which an ether group is introduced or a rawmaterial having a bent structure such as a meta-isomer is used isgenerally employed and superior. This agrees with the tendency ofsoluble polyimide compositions and is one of the reasons why solublepolyimides are advantageous in the present invention. It should benoted, however, that, since the glass transition temperature variesdepending on the combination of the components constituting thepolyimide, it is preferred that the polyimide having the aforementionedpreferred glass transition temperature be selected from those polyimideswhich contain at least one component having a carbonyl group and/or anether group as aromatic ring-linking group in the molecular skeleton. Incases where the main chain of the polyimide has a carbonyl group, sincethe carbonyl group and the aromatic ring(s) are highly flat, interactionwith a substance comprising a π-conjugated carbon is likely to takeplace. In addition, in cases where the main chain of the polyimide hasan ether group, since its loan pair contributes to stabilization of theinteraction and also increases the flexibility of the polyimidestructure, such a polyimide is preferred. Further, a raw material havinga sulfonic group or carboxyl group may be used as appropriate. Suchpolar groups are advantageous in that they can stabilize the electrodeactive substance and, at the same time, can improve the adhesion(attachment) with the current collector metal. Such polyimides arecommercially available, and examples of particularly suitablecommercially-available solvent-soluble polyimide products includeQ-VR-0756 (a block copolymerized polyimide produced from a raw materialhaving a carbonyl group and an ether group) and Q-VR-0757 (a blockcopolymerized polyimide produced from a raw material having a carbonylgroup and an ether group and a raw material having a carboxyl group),which are manufactured by PI R&D Co., Ltd.

Use of a polyimide as the binding agent (binder) of a lithium battery,particularly of a negative electrode, is described in the PatentDocuments 1 and 2; however, since those conventional polyimides proposedas the binding agents have an extremely high glass transitiontemperature (Tg) and such polyimides are rigid, the flexibility ofelectrodes is low and there are concerns in terms of the adhesionagainst detachment and the like; therefore, it is impossible to producea battery having practical reliability. Further, in conventionalpolyimides, since condensation curing of remaining amic acid isrequired, the electrodes must be dried under such a severe conditionthat the drying is carried out at a high temperature of at least about350° C. for not less than 2 hours. Consequently, there arise problemsof, for example, oxidative degradation of the current collector, so thatsuch conventional polyimides are difficult to be used as a binder evenin general batteries. However, the soluble polyimide used in the presentinvention does not have the aforementioned problems. Furthermore, fromthose examples in Patent Documents 1 and 2 which do not employ awidely-used material such as an active substance used in general lithiumbatteries, and particularly, also from the fact that the conductivematerial and the like employed therein for the positive electrode areused without a treatment with polyimide, it is judged that conventionalpolyimides are difficult to be used in general lithium batteries.

The conductive agent according to the present invention contains, as themain component, a reaction product between the aforementionedπ-conjugated carbon material and the soluble polyimide. The reaction canbe carried out by heating the π-conjugated carbon material and polyimidesolution at a temperature not lower than 80° C., preferably not lowerthan 100° C. and not higher than 180° C., preferably not higher than150° C., for a duration of preferably not less than 2 hours, morepreferably not less than 3 hours. There is no upper limit on thereaction time, but unnecessarily long heating is meaningless and simplyleads to an increased cost; therefore, the reaction time is usually notlonger than 8 hours. The mixing ratio of the π-conjugated carbonmaterial and the soluble polyimide at the time of the reaction is notparticularly restricted; however, the amount of the soluble polyimide(not including solvent) is preferably about 10 to 200 parts by weight,more preferably about 50 to 150 parts by weight, with respect to 100parts by weight of the π-conjugated carbon material. Further, as thesolvent employed in the reaction, the solvent used in thepolycondensation in the polyimide production may be employed as it is.That is, the soluble polyimide is produced in the solvent bypolycondensation and, therefore, obtained in the form of a solution. Thethus obtained solution may be used as it is. The aforementionedcommercially available soluble polyimide products are also sold in theform of a solution; therefore, such a product may be used as it is forthe reaction. The concentration of the polyimide in the solublepolyimide solution used for the reaction is not particularly restricted;however, it is usually about 10% by weight to 30% by weight. It ispreferred that the reaction be carried out with stirring, thereby areaction product can be obtained in the form of particles (hereinafter,this particulate reaction product may be referred to as “conductiveagent toner”). Although not bound by a theory, it is believed that thesurface of π-conjugated carbon and polyimide molecules form an orbit bythe aforementioned reaction.

The conductive agent according to the present invention contains, as themain component, the aforementioned reaction product between aπ-conjugated carbon material and a soluble polyimide. The term “the maincomponent” means that the content of the reaction product exceeds 50% byweight, and the content is preferably not less than 90% by weight, morepreferably not less than 99% by weight, most preferably 100% by weight(the conductive agent consists of the reaction product alone). Theconductive agent may contain other substance(s) that does not inhibitthe effects of the present invention and does not adversely affect theelectrode performance, as long as its/their content is within the rangeof less than 50% by weight, preferably within the range of not higherthan 10% by weight.

The conductive agent according to the present invention is used in thepreparation of an electrode for secondary batteries such as lithiumbatteries and fuel cells. As the battery, a secondary battery ispreferred and a lithium battery is particularly preferred. The electrodemay either be a positive electrode or a negative electrode.

The electrode according to the present invention can be prepared in thesame manner as conventional electrodes, except that the aforementionedconductive agent according to the present invention is used as theconductive agent. That is, the electrode according to the presentinvention is formed by coating a composition containing a conductiveagent and an electrode active substance onto a current collector. As thecurrent collector, as in the conventional method, a metal is preferablyused. Preferably, the electrode can be prepared by coating a slurrycontaining the conductive agent according to the present invention andan electrode active substance onto such a current collector andsubsequently drying it. Here, since the polyimide also functions as abinding agent, it is not necessary to use a separate binding agent;however, it may be used, and it is preferred to use such a separatebinding agent since it is easier to uniformly coat the currentcollector. As the binding agent, those which are conventionally used forpreparation of electrodes, such as PVDF, may be used. It is noted herethat, although those conventional binding agents such as PVDF have aproblem in the adhesion with current collector metals as describedabove, in cases where such a binding agent is used together with theconductive agent according to the present invention, those problems inthe aforementioned prior arts do not occur because of the actions of thesoluble polyimide.

The composition of the slurry to be coated onto the current collector isnot particularly restricted; however, with respect to 100 parts byweight of the electrode active substance, the amount of the conductiveagent is preferably 0.2 to 20 parts by weight, more preferably 1 to 5parts by weight, and the amount of the binding agent is preferably 0 to20 parts by weight, more preferably 1 to 5 parts by weight. In addition,the concentration of the conductive agent in the slurry is notparticularly restricted; however, it is usually 0.1% by weight to 10% byweight, preferably about 0.5% by weight to 3% by weight.

In cases where the present invention is applied to, for example, alithium secondary battery, as the positive electrode material (activesubstance), Li-containing composite oxides represented by thecomposition formula Lix MO₂ or Liy M₂O₄ (wherein, M is a transitionelement; 0<x≦1, 0<y≦2) and the like are exemplified. Specific examplesof the Li-containing composite oxide include LiCoO₂, LiMnO₂, LiNiO₂,LiCrO₂ and LiMn₂O₄.

In cases where the present invention is applied to, for example, alithium secondary battery, examples of the negative electrode material(active substance) include carbon compounds such as graphite, hardcarbon and coke. In cases where the conductive agent toner according tothe present invention causes lithium intercalation reaction and,therefore, has a battery capacity, the battery may also be designed withan addition of the battery capacity to the total capacity.

The positive electrode of the battery according to the present inventionis prepared by, for example, mixing a positive electrode activesubstance and, as required, a conductive agent toner into a solution inwhich PVDF is dissolved in an organic solvent such as NMP to produce aslurry and subsequently coating the slurry onto a current collectormetal by the doctor blade method, followed by drying of the organicsolvent by evaporation.

The negative electrode of the battery according to the present inventionis prepared by, for example, mixing a negative electrode activesubstance and, as required, a conductive agent toner into a solution inwhich PVDF is dissolved in an organic solvent such as NMP to produce aslurry and subsequently coating the slurry onto a current collectormetal by the doctor blade method, followed by drying of the organicsolvent by evaporation.

The present invention also provides a battery, preferably a secondarybattery, particularly preferably a lithium battery, which comprises theaforementioned electrode according to the present invention. Except thatthe aforementioned electrode is used, a well-known battery structure maybe employed.

In the battery according to the present invention, instead of using, asthe conductive agent, a conventional π-conjugated carbon material as itis, by employing a conductive agent toner, good binding property of theπ-conjugated carbon materials, as well as superior attachment betweenthe π-conjugated carbon materials and the powder of positive electrodeactive substance, are attained. Consequently, even with repeated chargeand discharge cycles, the conductive agent hardly detaches from thepositive electrode active substance; therefore, the battery capacity isless likely to be reduced. Further, since a network of the conductiveagents stably exists, the electrical resistance at the electrode partscan be stabilized, so that the reliability and safety as a battery canbe ensured.

EXAMPLES

The present invention will now be described in more detail by way ofexamples thereof; however, the present invention is not restricted atall to the following examples. The present invention can be carried outby modifying it as appropriate within the range in which the gist of thepresent invention is not modified.

Example 1 1. Preparation of Conductive Agent Toners (1) Preparation ofConductive Agent Toner 1

Loaded to a commercially available dispersion kneader machine (T. K.Hivis Disper Mix Model 3D-5 (manufactured by PRIMIX Corporation)) were1,000 g of Ketjen Black EC600JD (manufactured by Lion Corporation), 1500g of Q-VR-0756 (manufactured by PI R&D Co., Ltd.; 20%N-methyl-2-pyrrolidone (hereinafter, referred to as NMP) solution) as apolyimide varnish containing soluble polyimide, and 300 g of NMP as adilution solvent. After pre-kneading this mixture by hand mixing, theresultant was kneaded for 2 hours at 100 rpm to obtain a slurry. Afterfurther loading 500 g of NMP as the dilution solvent, the resultingmixture was heated to 100° C. and stirred for 5 hours to obtainconductive agent toner 1.

(2) Preparation of Conductive Agent Toner 2

Conductive agent toner 2 was prepared under the same conditions as inthe preparation of the conductive agent toner 1, except that Denka Black(manufactured by Denki Kagaku Kogyo K. K.) was employed as acetyleneblack in place of Ketjen Black EC600JD used in the preparation of aconductive agent toner 1.

2. Preparation of Lithium Battery

-   -   (1) Preparation of Positive Electrode

Lithium-cobalt composite oxide (LiCoO₂) as a positive electrode activesubstance, the conductive agent toner 1 as a conductive agent, and PVDF#1300 (manufactured by Kureha Corporation) were dispersed in NMP at asolid content weight ratio of 94:1:5 (the active substance:theconductive agent:the binder) to obtain a slurry (solids concentration of45% by weight). Thereafter, the thus obtained slurry was coated onto oneside of an aluminum foil used as a positive electrode current collectorby the doctor blade method, and NMP was dried at 120° C. in an oven toobtain a positive electrode.

(2) Preparation of Negative Electrode

A hard carbon, Carbotron P (manufactured by Kureha Corporation), as anegative electrode active substance, the conductive agent tonner 1prepared in the Example 1 (Preparation of conductive tonner agent 1) asa conductive agent, and PVDF #1100 (manufactured by Kureha Corporation)were dispersed in NMP at a solid content weight ratio of 91:1:8 (theactive substance:the conductive agent:the binder) to obtain a slurry(solids concentration of 50% by weight). Thereafter, the thus obtainedslurry was coated onto one side of a copper foil used as the negativeelectrode current collector by the doctor blade method, and NMP wasdried at 120° C. in an oven to obtain a negative electrode.

(3) Preparation of Electrolyte Solution

LiPF₆ was dissolved at a ratio of 1 mol/L in an equal-volume mixedsolution of ethylene carbonate and dimethyl carbonate to prepare anelectrolyte solution.

(4) Preparation of Battery

The above-prepared positive and negative electrodes and the electrolytesolution were used to prepare a cylindrical first battery MP1 (batterydimension: 14.2 mm in diameter; 50.0 mm in length). Here, used as theseparator was a polypropylene-made microporous thin film having ionpermeability (manufactured by Polyplastics Co., Ltd.; trade name “CellGuard 3401”).

FIG. 1 is a cross-sectional view of the thus prepared first battery MP1,and the first battery BA1 shown in the figure comprises a positiveelectrode 1, a negative electrode 2, a separator 3 which insulates theseelectrodes, a positive electrode lead 4, a negative electrode lead 5, apositive electrode external terminal 6, a negative electrode can 7 andthe like. The positive electrode 1 and the negative electrode 2 arehoused in the negative electrode can 7 in such manner that they arerolled in a spiral fashion via the separator 3 into which theelectrolyte solution is injected. The positive electrode is areconnected to the positive electrode external terminal 6 via the positiveelectrode lead 4, and the negative electrode 2 is connected to thenegative electrode can 7 via the negative electrode lead 5, therebyenabling chemical energy generated inside the first battery BA1 to bedrawn out as electric energy to the outside.

Example 2

A second battery MP2 was prepared in the same manner as in Example 1,except that the conductive agent toner 2 was employed as the conductiveagent used in the preparation of the positive and negative electrodes.

Comparative Example 1

A third battery CP1 was prepared in the same manner as in Example 1,except that Ketjen Black EC600JD was used as it is as the conductiveagent used in the preparation of the positive and negative electrodes.

Test Examples 1. Impedance of the Positive Electrode Surface

For each of the positive electrodes prepared in Example 1 andComparative Example 2, the impedance (Cole-Cole plots per 1 cm²) in theplanar direction in dried condition was measured. The results are shownin FIG. 2.

While the resistance component was 2.6 kΩ at the maximum for thepositive electrode (MP1) prepared using the conductive tonner agent, theresistance component of not less than 3 Id) was present over the entirerange for Comparative Example (CP1). In addition, in the electrodeaccording to the present invention whose resistance component in theplanar direction is small, the loss of voltage and electric power issmall during charge and discharge, so that the battery performs with noproblem. In contrast, since Comparative Example has a large resistance,it is believed that there would be problems of loss in the electricpower, side reactions and the like.

2. Cycle Characteristics of Each Battery

For each of the batteries prepared in Examples 1 and 2 and ComparativeExample 1, after charging the batteries at a charging current of 60 mAto a charge termination voltage of 4.2 V, the batteries were subjectedto a cycle test, where one cycle was defined as the step of dischargingat a discharging current of 200 mA to a discharge termination voltage of2.5 V, to examine the cycle characteristics of each battery. In thebatteries according to the present invention, MP1 and MP2, in which areaction product between the polyimide and the nanocarbon (Ketjen Blackor acetylene black) was used as the conductive agent, since the bindingproperty of the nanocarbon and the adhesion between the nanocarbon andthe current collector metal were superior, the electrode material hardlydetached from the electrodes even with repeated charge and dischargecycles, and even at the 1,000th cycle at which the test was terminated,the capacity loss was 8% and 10% of the initial capacity, respectively.In contrast, in the comparative battery, CP1, the amount of the activesubstance and the like detached from the electrodes increased with thenumber of cycles, and at the 1,000th cycle, the capacity loss was aslarge as 26% of the initial capacity.

3. Safety Test

The safety of each battery, MP1, MP2 and CP1, was examined by a simpletest method in which each battery was heated in an oven from roomtemperature to 200° C. While the battery cans of the batteries accordingto the present invention, MP1 and MP2, did not exhibit any change evenwhen they were heated to 200° C., that of the comparative battery, CP1,blew its cap off at the point where it was heated to 150° C. due to anincreased inner pressure. From these results, it is seen that, while thebatteries according to the present invention, MP1 and MP2, are highlysafe, the comparative battery, CP1, is at the risk of being damaged orrupturing when the battery temperature becomes abnormally high and,therefore, has a problem from the safety standpoint.

Although a cylindrical battery was exemplified in the above Examples,there is no particular restriction on the battery shape; therefore, inaddition to the cylindrical type, the present invention is applicable tolithium secondary batteries of various shapes, such as those of flattype and block type.

INDUSTRIAL APPLICABILITY

According to the present invention, a highly reliable secondary battery,as well as an electrode and a conductive agent used therefor, whichsecondary battery has a long cycle life and also is less likely to bedamaged or rupture even when the battery temperature becomes abnormallyhigh, are provided.

DESCRIPTION OF SYMBOLS

-   -   1: Positive electrode    -   2: Negative electrode    -   3: Separator    -   4: Positive electrode lead    -   5: Negative electrode lead    -   6: Cap    -   7: Stainless container

1. A method for producing a conductive agent for a battery electrode,said method comprising: mixing a π-conjugated carbon material and asolution of a soluble polyimide to give a mixture having a mixing ratioof 10 to 200 parts by weight of the soluble polyimide (not includingsolvent) with respect to 100 parts by weight of the π-conjugated carbonmaterial; and heating said mixture of the π-conjugated carbon materialand the solution of the soluble polyimide before mixing the π-conjugatedcarbon material and the soluble polyimide with an electrode activesubstance, wherein said heating is carried out at a temperature of notlower than 80° C. and not higher than 180° C. for not less than 2 hours,and wherein the polyimide contains at least one component comprising aunit having a carbonyl group and/or an ether group as an aromaticring-linking group in the molecular skeleton.
 2. The method according toclaim 1, wherein said electrode is for a secondary battery.
 3. Themethod according to claim 2, wherein said electrode is a positiveelectrode or a negative electrode of a lithium battery.
 4. The methodaccording to claim 1, wherein said polyimide has a glass transitiontemperature of not higher than 270° C.
 5. The method according to claim1, wherein said π-conjugated carbon material is at least one selectedfrom the group consisting of Ketjen Black, acetylene black, carbonnanotube and other carbon materials having a primary particle diameterof not greater than 10 nm.
 6. The method according to claim 5, whereinsaid π-conjugated carbon material is at least one selected from thegroup consisting of Ketjen Black, acetylene black and carbon nanotube.7. The method according to claim 1, wherein the soluble polyimide has aweight-average molecular weight of 30,000 to 200,000.
 8. The methodaccording to claim 1, wherein said mixing ratio of the π-conjugatedcarbon material and the soluble polyimide in the mixture is 50 to 150parts by weight of the soluble polyimide (not including solvent) withrespect to 100 parts by weight of the π-conjugated carbon material. 9.The method according to claim 1, wherein said heating is carried out ata temperature of not higher than 150° C.
 10. The method according toclaim 1, wherein said heating is carried out at a temperature of notlower than 100° C.
 11. The method according to claim 1, wherein saidheating is carried out at a temperature of not lower than 100° C. andnot higher than 150° C.
 12. The method for producing an electrode for abattery, said method comprising coating a composition containing theconductive agent according to claim 1 and an electrode active substanceonto a current collector.
 13. The method according to claim 12, whereinthe amount of the conductive agent is 0.2 to 20 parts by weight withrespect to 100 parts by weight of the electrode active substance. 14.The method according to claim 12, wherein said composition furthercontains a binder.
 15. The method according to claim 12, wherein saidelectrode is a positive electrode of a lithium battery.
 16. The methodaccording to claim 12, wherein said electrode is a negative electrode ofa lithium battery.
 17. A battery comprising an electrode produced by themethod according to claim
 12. 18. The battery according to claim 17,wherein said battery is a secondary battery.
 19. The battery accordingto claim 17, which is a lithium battery.